Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Ionomeric resins (ionomers) are copolymers of an olefin such as ethylene and an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid, and optionally softening monomers, in which some portion of the acidic groups in the copolymer is neutralized with metal ions such as sodium or zinc. Ionomers are thermoplastic resins exhibiting enhanced properties, e.g. improved resilience, stiffness or softness, toughness, durability, etc. for golf ball cover construction over balata (see below). As a result of their resilience, toughness, durability and cut resistance, various ionomeric resins sold by E. I. du Pont de Nemours & Company of Wilmington, Del. (DuPont) under the trademark “Surlyn®” and by the ExxonMobil Chemical Corporation of Houston, Tex. under the tradenames “Escor™” and “lotek™” have become materials of choice for the construction of golf ball covers instead of the traditional balata (natural or synthetic rubber) covers. The softer balata covers, although exhibiting enhanced playability, lack the durability and cut resistance desirable for repetitive play. In addition to their improved durability, harder covers or mantles tend to minimize ball spin and maximize the straightness of the ball's travel off the clubface. Some golfers prefer golf balls that maximize distance and provide low spin characteristics. Thus, it is desirable to continue developing materials for golf balls with increased hardness and stiffness.
Current commercial ionomers derived from dipolymers have properties that vary according to the type and amount of metal cations, molecular weight and composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups). Ionomers can also be modified by the addition of comonomers to modify their physical properties. For example, terpolymers made from an olefin such as ethylene, an unsaturated carboxylic acid and other comonomers such as alkyl (meth)acrylates provide “softer” resins, which can be neutralized to form softer ionomers.
Approaches to providing harder ionomers include the use of relatively high percentages of the carboxylic acid moieties in the copolymer. Disadvantageously, ethylene acid copolymers with high levels of acid are difficult to prepare in continuous polymerizers because of monomer-polymer phase separation. This difficulty can be avoided by use of “co-solvent technology” as described in U.S. Pat. No. 5,028,674 or by employing somewhat higher pressures than those at which copolymers with lower acid can be prepared. Despite these expedients, there is an inherent limit to the amount of carboxylic acid that can be incorporated as a monomer in ethylene acid copolymers. In addition, such methods result in higher costs and more complex processes.
The properties of thermoplastic resins, such as ethylene acid copolymers and ionomers thereof, can also be modified by blending other components into a polymer melt. For example, U.S. Pat. No. 6,608,127 describes golf balls having covers comprising an ionomeric resin modified by the addition of terpene resin tackifiers and/or rosin ester tackifiers. Similarly, U.S. Pat. No. 6,371,869 describes golf balls having covers comprising an ionomeric resin modified by the addition of thermoplastic elastomers and terpene resin tackifiers and/or rosin ester tackifiers. The compositions used for golf ball covers in these patents are characterized as having Shore D hardness of from 40 to 65.
Thus, it is desirable to develop a material for golf ball covers, mantles, intermediate layers, etc., having a combination of hardness, stiffness and good scuff resistance with improved heat stability and melt processibility. It is particularly desirable to develop compositions of ethylene acid copolymers with low to medium acid content that exhibit hardness comparable to or exceeding that exhibited by high acid copolymers.
Accordingly, one object of this invention is to provide an ionomer composition that is harder and stiffer than typical ionomeric compositions.
A further object of this invention is to provide a golf ball with a hard cover and/or intermediate layers having good resilience and durability.
A further object of this invention is to provide a golf ball having a favorable combination of low spin, durability and distance-carrying flight characteristics.
A further object of the invention is to provide a golf ball that is easy to prepare having a hard and stiff cover and/or intermediate layers. A further object of the invention is to provide a replacement for metal and wood components in sporting equipment, particularly golf clubs.